For use with RCBS Gold Medal Match series dies. Due to variability in neck brass thickness, RCBS standard dies size the neck below your desired diameter and then pull an expander ball through to get the proper bullet tension. The Gold Medal sizing dies (Full Length and Neck) features a self-centering, changeable neck bushing that allows you to precisely set the amount of tension on the bullet. This reduces work hardening of the case neck brass, prolonging case life and increasing accuracy. Neck bushings are available in .001” increments from .189” to .365”.RCBS suggests that you have neck bushings one to two sizes above and below your target diameter.